1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for recording information on a recording medium that allows an overwrite operation, and on a non-overwrite medium.
2. Related Background Art
As apparatuses for optically recording information or reproducing information by irradiating a light beam onto an information recording medium, a read-only type apparatus for reproducing information from a read-only recording medium on which information is recorded in advance, a WORM (write once, read many) type apparatus for recording information pits by forming pits on a recording film by heat, an apparatus for changing the crystal state of a medium and recording information as a difference in reflectance, a rewritable type apparatus for recording information pits by changing the direction of magnetization of a perpendicular magnetic film, and the like are available.
FIG. 1 shows the arrangement of a rewritable optical modulation overwrite type magnetooptical disk apparatus of the above-mentioned apparatuses. Referring to FIG. 1, a magnetooptical disk 1 serves as an information recording medium, and is constituted by forming a magnetic film 2 on a transparent substrate such as glass, plastic, or the like. The magnetooptical disk 1 is mounted on the rotation shaft of a spindle motor 3, and is driven to rotate at a predetermined velocity by the spindle motor 3. An optical head 4 is arranged below the lower surface of the magnetooptical disk 1, and a bias magnet 13 is arranged above the upper surface of the disk 1 to oppose the optical head 4. In the optical head 4, a semiconductor laser 5 serving as a recording/reproducing light source is arranged. A light beam emitted by the semiconductor laser 5 is collimated by a collimator lens 6, is transmitted through a polarization beam splitter 7, and is incident on an objective lens 8. The incident light beam is focused by the objective lens 8, and forms a very small (micro) beam spot on the magnetic film 2 of the magnetooptical disk 1. When information is recorded, the light beam emitted by the semiconductor laser 5 is modulated in accordance with an information signal, and is irradiated onto an information track of the magnetooptical disk 1. Also, upon recording information, the bias magnet 13 applies a magnetic field in a predetermined direction to the magnetooptical disk 1, and a series of information pits are recorded by the application of the magnetic field and the irradiation of the light beam.
The light beam irradiated onto the magnetooptical disk 1 is reflected by the medium surface. The reflected light is incident on the polarization beam splitter 7 via the objective lens 8 again, and is reflected toward the beam splitter 9 side by the polarization surface of the splitter 7. In this manner, the light beam is split from the incident light from the semiconductor laser 5. In the beam splitter 9, the incident light beam is split into two light beams. One light beam is received by an optical sensor 11 via a sensor lens 10. The light-receiving signal of the optical sensor 11 is input to an AT.cndot.AF circuit (auto-tracking.cndot.auto-focusing control circuit) 12. The AT.cndot.AF circuit 12 generates a tracking error signal and a focusing error signal on the basis of the light-receiving signal. An objective lens actuator 14 is driven on the basis of the generated tracking error signal and focusing error signal to displace the objective lens 8 in the tracking and focusing directions, thus attaining tracking control and focusing control.
On the other hand, when information recorded on the magnetooptical disk 1 is to be reproduced, a light beam to be emitted by the semiconductor laser 5 is set to have reproduction power lower than recording power, and recorded information is reproduced by scanning the reproduction light beam onto a target track. More specifically, reflected light of the reproduction light beam from the disk surface is received by an optical sensor 16 via the polarization beam splitter 7, the beam splitter 9, and a sensor lens 15. The light-receiving signal of the optical sensor 16 is supplied to a reproduction signal processing circuit (not shown), and is subjected to predetermined signal processing, thereby reproducing the recorded information. Of course, in the reproduction mode, the reflected light of the reproduction light beam is received by the optical sensor 11, and the AT.cndot.AF sensor 12 performs tracking control and focusing control based on the light-receiving signal.
The recording process of the optical modulation overwrite method in the apparatus shown in FIG. 1 will be described below. Note that the optical modulation overwrite method is described in detail in, e.g., Japanese Laid-Open Patent Application No. 63-239637. The magnetic film 2 of the magnetooptical disk 1 consists of first and second magnetic layers which are exchange-coupled to each other. The coercive force at room temperature of the first magnetic layer is larger than that of the second magnetic layer, and the Curie temperature of the first magnetic layer is lower than that of the second magnetic layer. When information is recorded on this disk 1, the second magnetic layer with a higher Curie temperature is initialized in one direction, and thereafter, an overwrite operation is performed by intensity-modulating the laser beam from the optical head 4. In this case, the laser beam has two different laser powers, i.e., first and second laser power levels. The first laser power level is a power level (a power level PL that forms a low-temperature level state) which raises the temperature of the disk 1 to the Curie temperature of the first magnetic layer, and the second laser power level is a power level (a power level PH that forms a high-temperature level state) which raises the temperature of the disk 1 to the Curie temperature of the second magnetic layer.
More specifically, a laser beam is modulated between the two different power levels in correspondence with information. When a laser beam having the first laser power level (PL) is irradiated, magnetization of only the first magnetic layer with a lower Curie temperature disappears, and magnetization that appears at the irradiated portion in a later cooling process aligns in a direction stable with respect to the initialized second magnetic layer by exchange coupling with the second magnetic layer with a higher Curie temperature (erasing process). Subsequently, when a laser beam having the second laser power level (PH) is irradiated, magnetizations of the first and second magnetic layers disappear in the irradiated portion, and the magnetization of the second magnetic layer, which appears in a later cooling process, aligns in the direction of the bias magnetic field. The magnetization of the first magnetic layer aligns in a direction stable with respect to the direction of magnetization of the second magnetic layer by exchange coupling, thus recording information (recording process). In this manner, by selecting the first and second laser power levels in correspondence with information, the magnetization of the first magnetic layer aligns in the initialization direction with the first laser power level, and aligns in the direction of the bias magnetic field with the second laser power level, thus recording information. In this manner, the laser beam is controlled between the two different laser power levels, and upon recording, an overwrite operation is allowed independently of the magnetization state of the first magnetic layer before recording.
The magnetooptical disk apparatus described above with reference to FIG. 1 is an apparatus which performs an overwrite operation of the optical modulation method, as described above, and can record information without erasing information, so as to meet the requirement for high-speed information recording. In recent years, in order to record information at high density, pit edge recording that assigns meanings as information to the two edges of a recording pit is becoming popular. However, in such pit edge recording, each recording bit must be formed to have a desired length, and to have a symmetrical shape in its longitudinal direction.
In general, when recording is performed by lighting a laser beam in correspondence with a recording signal itself, each bit formed on the medium has a teardrop shape which widens in the diffusion direction of heat. More specifically, this phenomenon occurs due to thermal interference between adjacent bits, and this means that the method of lighting a light beam in correspondence with the recording signal cannot cope with the above-mentioned pit edge recording. In order to eliminate the influence of thermal interference, as shown in FIG. 2, a method of performing recording using a four-value multi-pulse recording waveform, i.e., a method of performing recording by lighting a laser beam using four power level values has been proposed. FIG. 2 shows, as an example, a laser lighting waveform when a 4T pattern is to be recorded.
Referring to FIG. 2, PL indicates the power level for forming a low-temperature level state (erasure) on a recording layer of an overwrite recording medium such as the above-mentioned magnetooptical disk 1, i.e., for executing an erasing process. When the medium is pre-heated by a light beam with the power level PL, a preheat effect can be obtained. PH1 and PH2 indicate the power levels for forming a high-temperature level state (recording) on the recording layer, i.e., for executing a recording process, and Pr indicates the reproduction power level with a constant value. PH1 is lighted for a 1.5T period, and PH2 is then pulse-lighted at 0.5T intervals. After PH2, a 1.0T cooling period is assured, and this period is normally called a trailing cooling gap. In this manner, in FIG. 2, the laser lighting operation is controlled using four values, i.e., PL, PH1, PH2, and Pr (Pb). The reason why PH2 is pulse-lighted after PH1 is to maintain the temperature of the recording medium to a predetermined value, and to prevent the medium from being overheated.
When the laser lighting operation is controlled in this manner, the above-mentioned influence of thermal interference can be eliminated, and variations in pit edge can be suppressed. Therefore, the laser beam control method shown in FIG. 2 can be suitably used in pit edge recording. Note that FIG. 2 shows the laser lighting waveform of the 4T pattern. Also, patterns with other lengths are formed as follows. When, for example, (1-7) modulation is used as the modulation method of recording data, the shortest bit length is 2T, and the longest bit length is 8T. Therefore, taking (1-7) modulation as an example, in order to form the shortest 2T pattern, only PH1 is lighted after PL in FIG. 2. To form a 3T pattern, PH1 is lighted after PL, and thereafter, only one PH2 pulse (one period of PH2) is lighted. To form a 4T pattern, a laser beam is lighted, as shown in FIG. 2. Similarly, to form 5T, 6T, 7T, and 8T patterns, three, four, five, and six PH2 pulses respectively follow PH1.
When the laser lighting operation is controlled by the above-mentioned control method to record information on an overwrite medium, a single medium may exhibit different recording characteristics depending on different setting values of PL. More specifically, as the value PL is set to be closer to a power level PHth immediately before the high-temperature level state begins, the leading edge of a recording bit becomes less sharp, and such a blunt leading edge increases jitter upon reproduction. This phenomenon occurs since the edge of a recording bit becomes sharper as it undergoes a larger temperature change. Also, the value of PL is determined by the characteristics of a medium to some extent. That is, if the power difference between the power level PHth immediately before the beginning of recording and a minimum erasing power level PLmin at which a recorded bit can be completely erased is small, the value PL must be set to be close to the power level PHth. Therefore, in this case, a sharp leading edge of a recording bit cannot be formed, resulting in large jitter upon reproduction.
On the other hand, if the power difference between the power level PHth immediately before the beginning of recording and the minimum erasing power level PLmin at which a recorded pit can be completely erased is large, the value PL can be separated from PHth. Therefore, in this case, a sharp leading edge of a recording bit is formed, and jitter upon reproduction can also be reduced. In this manner, since the value PL is determined depending on the medium characteristics, a sharp leading edge of a recording bit cannot be formed in a medium with characteristics with a small power difference between PHth and PLmin, resulting in large jitter upon reproduction.
When recording is performed on a non-overwrite medium, the influence of thermal interference can also be eliminated using the four-value multi-pulse recording waveform, as shown in FIG. 2. The laser control method for a non-overwrite medium is the same as that in the above description. For a non-overwrite medium, the power level for pre-heating a medium is called Pas. However, in a non-overwrite medium as well, a single medium has different recording characteristics depending on different values Pas as in an overwrite medium. More specifically, as the value Pas is set to be closer to a power level Pth immediately before the beginning of recording, a sharp leading edge of a recording bit cannot be formed, and jitter upon reproduction increases. This is because the edge of a recording bit becomes sharper as it undergoes a larger temperature change, as in the above-mentioned overwrite medium.
Also, in a non-overwrite medium, the value Pas is determined by the medium characteristics to some extent. More specifically, when the power difference between the power level Pth immediately before the beginning of recording and a maximum reproduction power Prmax at which a recorded pit is not erased is small, the value Pas must be set to be close to the power level Pth immediately before the beginning of recording. However, in this case, since Pas and Pth are close to each other, a sharp edge of a recording bit cannot be formed, resulting in large jitter upon recording. Conversely, when the power difference between the power level Pth immediately before the beginning of recording and the maximum power level Prmax at which a recorded pit is not erased is large, since the value Pas can be set to be separated from Pth, a sharp edge of a recording pit can be formed, and jitter upon reproduction can be reduced. As described above, in a non-overwrite medium as well, since the value Pas is determined depending on the medium characteristics, a medium having characteristics with a small power difference between Pth and Prmax suffers large jitter upon reproduction.
In order to solve the above-mentioned problems of the overwrite and non-overwrite media, a method of assuring a cooling gap before a four-value multi-pulse recording waveform, as shown in FIG. 3, is proposed. More specifically, FIG. 3 shows a laser lighting waveform used when a 4T pattern is recorded, as in FIG. 2. In FIG. 3, by assuring a 0.5T cooling gap (called a leading cooling gap) before PH1, the medium is cooled in advance to obtain an abrupt temperature change of the medium. Therefore, with this method, when an overwrite medium has a small difference between PHth and PLmin and PL must be set to have a value close to PHTh, or when a non-overwrite medium has a small difference between Pth and Prmax and Pas must be set to have a value close to Pth, since the medium is cooled before a bit is formed, an abrupt temperature change is obtained at the leading edge of a bit, and a sharp leading edge of a pit can be formed.
However, conversely, when PL is set to be a value separated from PHth in an overwrite medium, or when Pas is set to be a value separated from Pth in a non-overwrite medium, this method is not suitable in terms of stability of a recording signal since such an unerased portion may remain. That is, in this case, the recording waveform shown in FIG. 2 is suitable. As described above, in the conventional method, when information is recorded on an overwrite or non-overwrite medium using multi-value recording powers, since the recording characteristics vary depending on the medium characteristics, if recording is performed using an unsuitable recording waveform, jitter upon reproduction increases, or an unerased portion undesirably remains.